Horn loaded piezoelectric matrix printer drive method and apparatus

ABSTRACT

A method and apparatus for driving a matrix printer element is described which utilizes a piezoelectrically driven horn loaded driver. The horn is preferably tapered in the direction of the printing element and has a base area to which is attached a piezoelectric crystal excitation device. The piezoelectric crystal structure is driven by electric pulses which are coupled to the tapered horn to increase the impact velocity and stroke at the output end of the horn. The piezoelectric crystals may be controlled by the application of a particular pulse pattern to reduce resonance and insure an optimum deceleration of the crystal during rebound of the printing element.

TECHNICAL FIELD

This invention relates to piezoelectric drive systems in general and todrive means and apparatus for matrix printer elements in particular.

PRIOR ART

From ultrasonic technology, the use of horns to increase motional andvelocity amplitudes is known. (See W. P. Mason, "Physical Acoustics andthe Properties of Solids", publishers Van Nostrand, Princeton, N.J.(1958), page 157 ff). For this purpose, a resonance-tuned horn iscoupled to an acoustic source which may be a piezo crystal structure.When such a crystal is used as an acoustic source, it may be energizedby means of an electric current applied in a stationary sinusoidaloscillation. When this occurs, the horn is operated in resonance and theoptimum use of the energy is ensured. Such resonance-driven horns areused for ultrasonic drills, ultrasonic bonding, and other applications.

In conventional mechanical matrix printers, the kinetic and printenergies are electromagnetically generated. In the case of large strokesof the print element, a repetition rate exceeding 2 kilohertz is notusually obtainable at a velocity which is sufficient to produce severalcopies. This is due to the fact that currents and current densities areallowed to assume limited values only. Generally, the time available fora print cycle is less than 400 microseconds (at typical travels of amatrix print element of 0.5 to 0.8 mm). Thus, velocities of 2 to 5m/sec. are necessary for energizing the print element.

In the German Offenlegungsschrift No. 2524854 the use of piezo crystalstructures for matrix printing is described in principle. However, theelongation velocity of the piezoelectric drive element is limited by thebreaking point of the piezo ceramic material. Limit values for modernpiezo ceramics can reach velocities of 0.2 to 0.4 m/sec. Even in thecase of mechanical bias, these values cannot be increased further atadequate electric voltages. The piezo elongations obtainable are verysmall (5 to 10 μm at a crystal length of 5 cm). These values areinsufficient for optimum impact processes such as are necessary formatrix printers, for example.

The simplest way of increasing the stroke is to use long piezo crystalelements, but the disadvantages of this would be unhandiness, highprice, high capacity, and ever increasing oscillation periods of thesystem. Even if these disadvantages may be tolerated in the interest ofa larger elongation, the impact velocities would remain unaffected,i.e., they cannot be increased by such measures.

From the German Offenlegungsschrift No. 2 342 021, a matrix print headfor typewriters is known, wherein different electric fields are appliedin rapid succession to elongated piezoelectric transducers which act ondot-generating, adjacent print elements. The structure described in saidOffenlegungsschrift does not permit a sufficient elongation (formultiple forms, i.e., copies) of the piezo crystal. In the case of anoptimum adaptation of the mass, the mass of the printing needle wouldhave to correspond to the effective mass of the piezo crystal, i.e., themass of the needle would have to be very large. This, in turn, wouldhave the disadvantage that high printing frequencies would beimpossible, because the crystal elongation velocity is below 0.1m/sec--assuming adequate crystal lengths.

Moreover, it has been proposed to provide a matrix printer withpiezoelectrically operated printing needles with a buckling spring whichis deflectable in the case of an electrically controlled elongation of apiezo-crystal structure, the deflection of the buckling spring beingtransferrable to a printing needle coupled to it.

This structure has the disadvantage that a buckling spring is soft andpermits only low printing forces and limited printing frequencies. Inaddition, the buckling process subjects the material to substantialstresses.

OBJECTS OF THE INVENTION

In light of the foregoing problems in the prior art, it is the object ofthis invention to provide an improved drive system which, while usingthe usual piezo crystal structures, permits increasing the impactvelocity and/or a controlled stroke.

BRIEF DESCRIPTION OF THE DRAWINGS

Practical examples of the preferred embodiments of the invention areshown in the drawings and will be described in greater detail below.

FIG. 1 is a diagrammatic representation of a piezoelectrically drivenhorn for driving the printing needle of a matrix printer.

FIG. 2 is a diagrammatic perspective view of a matrix-type arrangementof several horns.

FIG. 3A is a diagrammatic representation of a control pulse with theamplitude represented as a function of time.

FIG. 3B is a diagrammatic representation of the elongation of the piezocrystal structure as a function of time in the case of energization inaccordance with FIG. 3A.

FIG. 3C is a diagrammatic representation of the elongation of the horntip as a function of time in the case of energization in accordance withFIG. 3A.

FIG. 4A is a diagrammatic representation of the energization of thepiezo crystal structure by means of a voltage step with the amplitude asa function of time.

FIG. 4B is a diagrammatic representation of the elongation of the piezocrystal structure as a function of time in the case of energization inaccordance with FIG. 4A.

FIG. 4C is a diagrammatic representation of the elongation of the horntip as a function of time in the case of energization in accordance withFIG. 4A.

FIG. 5 is a diagrammatic representation of a step-shaped voltage coursefor energizing the piezo crystal structure to avoid resonance phenomena.

FIG. 6A is a diagrammatic representation of the voltage course forenergizing the piezo crystal structure to obtain optimum velocityrelations on the horn tip.

FIG. 6B is a diagrammatic representation of the elongation of the piezocrystal structure as a function of time in the case of energization inaccordance with FIG. 6A.

FIG. 6C is a diagrammatic representation of the elongation of the horntip as a function of time in accordance with an energization of FIG. 6A.

FIG. 7 is a diagrammatic representation of an arrangement for generatingink droplets in ink jet printers.

FIG. 8 is a diagrammatic representation if an arrangement for generatingink droplets in ink jet printers using the piezoelectrically driven hornin accordance with the invention.

DETAILED SPECIFICATION

FIG. 1 shows a piezoelectrically driven horn for driving the printingneedle of a matrix printer. The horn is referred to as 1. Horn 1 ispreferably tapered towards the tip, following an exponential course.Deviations from this course affect the pulse transfer function. The hornpreferably consists of solid material of the kind generally used inultrasonic technology, preferably aluminum and at best titanium alloys.On the base surface 1A of the horn 1, a bundle 7 of piezoelectriccrystal elements 2, 3, 4, and 5 is arranged which by means of a clampingstud 7A and a clamping plate 16 which are rigidly connected to thehorn 1. The piezo crystal structure 7 is energized by electric pulses.The pulses are applied to the terminals 11 and 15, lines of which leadup to the individual pole connecting faces of the piezoelectric crystalelements.

Upon energization, the piezo crystal structure 7 is subject todeformation which, via the base surface 1A of the horn 1, propagatesinto the horn 1. As a result of the generally known horn transferproperties, the elongation of the piezo crystal structure 7 istransformed into an increase in stroke and velocity on the horn tip 1B.This means that smaller elongations of the piezo crystal structure 7affect the horn tip to create larger strokes. Also, compared to theelongation velocity of the piezo crystal structure, the velocity of thehorn tip is higher.

The horn tip is freely movable in a longitudinal direction. The base orclamping plate 16, serving to fix the piezo crystal structure 7 to thehorn base surface 1A, is permanently connected to the supporting part 18either by means of a screw joint, a welded or an adhesive joint, or bysome other means. Upon application of a pulse, the horn tip 1B iselongated in the direction of the arrow, acting by impact coupling onthe printing needle 19 which is thus accelerated in the direction ofprint. As the mass of the printing needle is very small in comparison tothe effective mass of the horn tip which is substantially greater, theimpact additionally leads to an increase in velocity in accordance withthe momentum conservation law.

The printing needle is guided in a known manner (e.g., in printers suchas the IBM 3284 or 3286) in a flexibly suspended guide. This guide doesnot form part of the subject matter of the invention and is, therefore,not shown or described in detail. Further means for coupling the horntip to the printing needle would be, for example, a fixed couplingobtained by soldering, welding, etc.

The individual piezo crystal elements 2 to 5 are commercially available.Such a piezo crystal element is provided with two pole connecting faces,e.g., 4A and 4B. Upon application of a corresponding control voltage tothe pole connecting faces, the length of the element is changed. Theindividual piezo crystal elements 2 to 5 are connected in such a mannerthat similar pole connecting faces are arranged adjacent to each other.They are thus electrically paralleled and mechanically series-connectedwith regard to their effective elongation. The complete piezo crystalstructure 7 should always comprise an even number of piezo crystalelements. All pole connecting faces associated with a negativepolarization polarity are connected to the positive pole + of a voltagesource via the lines 12 and 13. All pole connecting faces of a positivepolarization polarity are connected to the negative pole - of saidvoltage source via the lines 8, 9, and 10. If the piezo crystalstructure 7 consists of an even number of piezo crystal elements, theterminals for the positive pole + of the voltage source, which arearranged inside said structure, are shielded in the case of externalpole connecting faces with a positive polarization polarity, which areconnected to the negative grounded pole - of the voltage source.

Upon application of a control pulse to terminals 11 and 15, theeffective elongations of the individual piezo crystal elements 2 to 5cumulate in the arrow-marked direction. This cumulative elongation istransferred to the horn 1 and is transformed, as described above, in thedirection of the horn tip 1B.

It is pointed out that it is also possible to use piezo crystal elementsin which the polarization direction and the electric field areperpendicular to each other. In such a structure the piezo crystalelement in the direction of polarization is subject to smaller lengthchanges than in cases where the direction of polarization corresponds tothe direction of the electric field.

It is conducive to the operation of the structure in accordance with theinvention that disk-shaped piezo crystal elements be used and stackedupon each other. Such elements should be tapered in the direction of thehorn tip and thus are adapted to the transfer characteristic of thehorn. The length of the piezo crystal structure 7 governs the lengths ofthe edges of the pulses emitted on the horn tip and thus also the timeavailable for the impact. A length of 5 cm corresponds to a typicalorder of magnitude. The total length of the piezo stack 7 is derivedfrom the relation that the transit time in the piezo crystal structureshould exceed the impact time by L/c (where L=length of the piezo stackand c=speed of sound in the material of the horn.

The horn length L is to be chosen in such a manner that the elongation(stroke) on the horn tip is high in relation to the elastic deformationsof horn tip and printing needle upon impact and high in relation to thepeak-to-valley heights of the impact faces concerned.

The physical-mathematical principles of amplitude and velocitytransformation on horns are known, for example, from E. Eisner "Journalof the Acoustical Society of America," Volume 41, page 1126 (1967).

In accordance with this, the velocity amplitude on the horn tip isessentially a function of the horn parameter resulting from the inputface/output face ratio (input face=base surface 1A; output face=face 1Bof the horn tip). With an exponentially tapered horn having an inputface/output face ratio of 1 cm² /1 mm², the velocity on the horn tipincreases by the factor 5 to 6.

As previously mentioned, the printing needle is driven by impactcoupling as a result of the horn tip bouncing forward. The printingneedle, as described above, is flexibly guided in a conventional manner,to accelerate the restoring motion and to ensure permanent contact ofthe faces in between impacts. The surfaces of both impact elements, horntip and printing needle, should have Vickers hardnesses exceeding 600kp/mm², to prevent permanent deformations.

In conventional matrix printers, a character to be printed is generatedby several print wires. Said print wires can be arranged either onebelow the other in a column or in matrix form. To realize, for example,a matrix-type printer arrangement with several printing needles, theindividual horns 1 associated with each print wire each must also bearranged in matrix form, as shown in FIG. 2. Each horn 1-1, 1-2, 1-3,1-4 to 1-20 carries its own piezo crystal structure 7-1, 7-2, 7-3 to7-20 on its base surface. The totality of the horns with associatedpiezo crystal structure is arranged in matrix form on a common piezocrystal structure 24. This piezo crystal structure 24, analogous to thepiezo crystal structure 7 of each horn 1, consists of several piezocrystal elements 20 to 23 which are similarly connected and which bymeans of a pulse on terminals 28 and 29 are induced to a basicdeformation. This basic deformation is selectively superimposed by theelongation of the piezo crystal structure 7-1 to 7-20 of each horn 1-1to 1-20, provided the structure is induced to a deformation by means ofan electric pulse. For simplicity's sake, the electrical connections forthe individual piezo crystal structures 7-1 to 7-20 are not shown.

In this manner a maximum stroke made up of the deformation of the piezocrystal structure of the horn proper and that of the common piezocrystal structure 24 is obtained on the tip of a selected horn. Thecommon piezo crystal structure 24, in turn, is mounted on a rigidlyfixed base plate 25. The individual horns are held by bolts (not shown)which from the rear side of the base plate 25 extend through bores inthe individual piezo crystal elements of the stack 24 and through boresin the individual elements of the piezo crystal structures 7-1 to 7-20specific to each horn. For geometric reasons, the totality of the horntips must be merged on a surface which is smaller than the surface ofthe common piezo crystal structure 24. This surface transformation isnecessary because the arrangement of the horn tips must be adapted tothe matrix of the print wires (not shown). To force such a surfacetransformaton (for the center line of the individual horns, a pathdeviating from a straight line is necessary), the individual horn tipsare led through the guide holes 27 of a horn tip guide element 26, whichare arranged in matrix form. From guide element 26 said horn tips act ontheir associated printing needles (not shown) by impact coupling, forexample.

For a perfect print, final velocities on the horn tip of 5 m/sec. arenecessary. The stroke associated with this velocity must be of the orderof about 20 μm, which is sufficient for the impact process. The strokeobtained must considerably exceed the elastic deformations which areencountered upon impact both on the horn tip and on the printing needle.In the case of a horn with a stroke and velocity transforming function,the piezo crystal elements are subjected to less mechanical wear than ifthey had to provide the required stroke themselves. As a result, therisk of mechanical depolarization of the piezo crystal elements iseliminated.

Print processes require clearly controlled stroke and velocitycharacteristics of the horn tip. At high printing frequencies, the horntip must be available for a new printing step without ensuing freeoscillations. To achieve this, the individual horns are energized bymeans of a control pulse and a control pulse program, respectively,which are applied to the piezo crystal structure.

FIG. 3A shows such a control pulse with the amplitude being a functionof time. (Time=t, amplitude=u). In time relation to this representation,FIG. 3B shows the elongation xc of the piezo crystal as a function ofthe time t, and FIG. 3C shows the elongation of the horn tip xh as afunction of the time t. The triangular stroke path in accordance withFIG. 3B may be theoretically explained by means of the article by W.Eisenmenger in the German journal "Acustica" 9, page 327, 1959.

Upon application of a pulse to the whole piezo crystal structure 7, thelatter is mechanically biased. As a result of this bias, so-calledstrain waves from the ends of the piezo crystal structure 7 extendlinearly into the crystal, leading to areas of increasing lineardeformation in the direction of the crystal center.

In comparison to the elongation of the piezo crystal structure inaccordance with FIG. 3B, the elongation of the horn tip is partlynegative and delayed in time as a result of the dispersion of the wavein the horn proper. The path of elongation of the horn tip in accordancewith FIG. 3C is an optimum one which can be obtained only if the pulsewidth of FIG. 3A has a particular value. If the pulse width exceeds oris less than this value, periodic elongations, whose amplitudes decreasemerely as a result of damping in the piezo crystal structure and in thehorn proper, rather than single elongations are encountered. Suchperiodic free oscillations are undesirable, because they do not permithigh printing frequencies. A high printing frequency necessitates thatthe horn tip is at rest before a new print process is started.

Ensuing free oscillations of this kind are also encountered when thepiezo crystal structure is merely energized as a result of a voltagestep in accordance with FIG. 4A.

In FIG. 4A the course of the voltage step is shown as a function of time(time=t; amplitude=u). In time relation to this representation, FIG. 4Bshows the elongation of the piezo crystal structure xc as a function ofthe time t, and FIG. 4C shows the elongation xh of the horn tip as afunction of the time t.

In the case of such step-shaped energization in accordance with FIG. 4A,the expansion in the crystal shows a permanent periodicity. In practice,the amplitudes occurring would assume lower values as a result ofdamping in the course of time. The path of elongation on the horn tip inaccordance with FIG. 4C is also subject to ensuing free oscillationswhich are merely influenced by damping. It is pointed out that in thiscase a periodicity in the amplitude course is not given because of thetransfer conditions in the horn.

In the ideal case (piezo crystal structure without attached horn), thisfavorable pulse width in accordance with FIG. 3A has a value of ##EQU1##

This favorable pulse width is desirable because it leads to clearelongation characteristics on the horn tip without detrimental echo orreflection effects.

If the horn tip is to be elongatable by x within the shortest time andsubsequently is to be at a complete standstill (without ensuing freeoscillations), a control pulse program in accordance with FIG. 5 shouldbe chosen for the piezo crystal structure (time=t; amplitude=u). Thisrepresentation shows a so-called double step in the control pulse,whereby the step size is to have a magnitude of 2(L/c) (L=length of thepiezo crystal structure; c=speed of sound in the horn material). Thisapplies to an ideal piezo crystal body without a horn coupled to it.Such control of the piezo crystal structure can be applied to particularadvantage in so-called servo systems which are intended to performaccurately controlled strokes in the minimum time.

For matrix printing, on the other hand, the horn tip must have aninitial velocity which is as high as possible over an adequate length ofstroke. In such a case it is advisable to control the piezo crystalstructure by means of pulses in accordance with FIG. 6A (time=t;amplitude=u). In time relation to this representation, FIG. 6B shows theelongation of the piezo crystal structure xc as a function of the timet, and FIG. 6C shows the elongation xh of the horn tip as a function ofthe time t. With such a course of the control variable (FIG. 6A) optimumvelocity relations are obtained on the horn tip (FIG. 6C). The voltagecourse for controlling the piezo crystal structure is characterized inthat a negative pulse of the width 2(L/c) is followed by a positivepulse of the width 4(L/c) and then again by a negative pulse of thewidth 2(L/c). (L=length of piezo crystal structure; c=speed of sound).

The elongation velocity of the piezo crystal structure corresponds tothe pitch of the edges in accordance with FIG. 6B. This representationthus shows that during the first negative pulse having a width of 2(L/c)the piezo crystal structure contracts at a relatively low velocityduring the positive pulse with the width 4(L/c). Subsequently, duringthe second negative pulse with a width of 2(L/c), the piezo crystalstructure again contracts at simple velocity. FIG. 6C shows theelongation of the horn tip as a function of time, which means that thehorn tip initially moves back at a low velocity, to subsequently expandat about three times that velocity in the direction of print. Then thehorn tip returns to its standstill position at low velocity, performinga short stroke. Use of the voltage course in accordance with FIG. 6A forthe control of the piezo crystal structure ensures that the printingneedle dynamically returns to its standstill position shortly before thepulse program is terminated.

A further advantage of a pulse-controlled horn is the high concentrationof kinetic energy in the region of the horn tip, which increases theeffectiveness of the energy transfer.

For impact operation (horn/printing needle) the pulse program can bemodified to compensate the effect of the printing needle bouncing back.By means of a correspondingly predetermined control pulse at the timewhen the needle bouncing back impacts the horn tip, a velocity opposedto that of the needle can be generated in the horn in such a manner thatthe two velocity components cancel each other. The use of such acompensating pulse ensures that after impact needle and horn tip aredynamically at rest. Such a compensating pulse can be empiricallydetermined as a function of the mass and velocity of bodies impactingeach other.

The difference between the representation of the elongation of anunloaded piezo crystal structure in comparison to the deflection on thehorn tip (see FIGS. 4A-3C, 4A-4C, 6A-6C) is due to the fact that theimpact waves in the horn, in addition to delays in the travel times andreflections, are subject to distortions.

The pulse widths shown in FIGS. 3A, 5, 6A are to be adapted to suchdistortions, empirically determining that pulse width at which the horntip at the end of the pulse does not continue to oscillate.

Even though the example of FIGS. 1 and 2 refers to a matrix printer, thesystem in accordance with the invention can also be used elsewhere.

The structure in accordance with the invention is also suitable for inkjet printers. In accordance with a known principle ("IBM Journal ofResearch and Development," 1977, page 2) an electric control pulse, asshown in FIG. 7, is applied to a piezo crystal element 35. As a resultof this pulse, the piezo crystal element is deformed in the arrow-markeddirection. The deformation is transferred to a fluid reservoir 36connected to the piezo crystal element, so that from a cannula system,with storage tank 38 connected to said reservoir, an ink droplet 36 isemitted at the exit opening. The fluid reservoir is separated from thepiezo crystal 35 by means of a membrane 40. The energy of a tiny dropletthus emitted is of the order of 5 erg and thus considerably below theenergy which is generally required for matrix printing.

In FIG. 8 the cannula system is again designated as 37 and the connectedstorage tank as 38. The tip of the piezoelectrically controlled horn 41in accordance with the invention, on whose base surface the piezocrystal structure 42 energized by a pulse (a pulse program) is arranged,leads into said cannual system.

The pulse control permits a more direct control of the pressureconditions in the cannula system than would be possible in thearrangement of FIG. 7. The usual reservations with regard to a dampedretraction of the membrane (FIG. 7) do not apply in this case. The useof the prior art reservoir with the membrane does not permit higherfrequencies during the generation of ink droplets. Higher frequenciesare necessary, however, to ensure a higher printing efficiency and abetter quality of print which can be obtained by means of the system inaccordance with the invention.

Having thus described by invention, what I claim as new, and desire tosecure by Letters Patent is:
 1. Piezoelectrically operated driveapparatus for matrix printer elements, comprising:a tapered, horn shapedbody of material having a base surface input end and a surface outputend located at opposite ends of said horn shaped body; a piezoelectriccrystal structure energizable by electric current attached to said basesurface input end of said horn shaped body; and means for applyingcontrolled electrical pulses being of rectangular form and of controlledduration and applied in a plurality of steps in which the voltage levelof said steps increases in amplitude to generate a controlled strokeresponse at the output end of said horn body.
 2. A method of operating apiezoelectrically controlled drive system for matrix printers having ahorn shaped transmission body having a base input surface on one end ofsaid horn shaped body and an output surface on the other end thereof anda peizoelectric crystal structure attached to said base input surfaceend of said horn, comprising steps of:energizing said piezoelectriccrystal means by means of an electrical pulse of rectangular form and ofcontrolled duration, said energizing being conducted in a plurality ofsteps during said pulse, the voltage level level of said stepsincreasing in amplitude to generate a controlled stroke response at theoutput surface of said horn body.
 3. The method as described in claim 2,further comprising:applying a sequence of negative, positive andnegative rectangular shaped pulses of controlled duration to saidpiezoelectric crystal for generating a high impact velocity at said hornshaped body output surface.